Conversion treatment solutions for aluminum and aluminum alloys may be roughly classified into chromate-type treatments and nonchromate-type treatments. Typical examples of chromate-type treatments are chromic acid/chromate treatments and phosphoric acid/chromate treatments. Chromic acid/chromate treatments came into practical application in about 1950, and are still widely used at present on, for example, the fin material of heat exchangers. The principal components of this type of conversion treatment solution are chromic acid (CrO.sub.3) and hydrofluoric acid (HF), and an accelerator may also be present. A film which contains some quantity of hexavalent chromium is formed.
The phosphoric acid/chromate conversion treatment is disclosed in U.S. Pat. No. 2,438,877. This conversion treatment solution is composed of chromic acid (CrO.sub.3), phosphoric acid (H.sub.3 PO.sub.4), and hydrofluoric acid (HF). The principal component of the resulting film is hydrated chromium phosphate (CrPO.sub.4.4H.sub.2 O). Since this film does not contain much if any hexavalent chromium, it is widely used at present as a paint undercoating treatment for beverage cans and the associated lid stock.
Nonchromate-type treatments are recognized in the art as a distinct category from the chromate-type treatment solutions explained above, and are exemplified by the invention disclosed in Japanese Patent Application Laid Open [Kokai] Number 52-131937 [131,937/77]. The treatment solution disclosed therein comprises an acidic (pH approximately 1.0 to 4.0) aqueous coating solution which contains zirconium or titanium or a mixture thereof as well as phosphate and fluoride. Treatment with the disclosed conversion treatment solution produces on the aluminum surface a conversion film whose main component is zirconium and/or titanium oxide. Although the absence of hexavalent chromium is an advantage of the nonchromate-type treatment solution, this type of treatment solution nevertheless suffers from a corrosion resistance and paint adherence inferior to those for chromate-type treatments.
Aluminum alloy, in sheet or coil form, is widely used after painting for beverage can lid material, i.e., can end stock. It is subjected to a conversion treatment in order to raise the corrosion resistance and paint adherence, and the phosphoric acid/chromate treatment is employed in almost all commercial can lid manufacturing in Japan.
The phosphoric acid/chromate conversion treatment of can end stock generally employs a treatment solution which contains 10.0 to 40.0 g/L phosphate ion, 2.0 to 4.0 g/L hexavalent chromium, and 0.7 to 1.5 g/L fluoride ion. At present, vinyl chloride paint is generally used to coat can end stock. Thus, the production of can ends normally includes a phosphoric acid/chromate treatment of aluminum alloy in coil or sheet form, followed by coating with a vinyl chloride paint and then forming.
A beverage can thus normally consists of a can end formed from aluminum alloy coil or sheet treated as described above and of a can body filled with, for example, juice or beer. Depending on its contents, the can may be subjected to sterilization at relatively high temperatures after filling. If it is, steam is formed from vaporization of the contents, the steam penetrates through the paint film, and the permeated steam then condenses at the interface between the paint film and conversion film. As a result, sterilization tends to reduce the adherence of the paint film. In particular, when a section of the can end is opened by the easy-open method, defects (enamel feathering) can be generated in the opened region due to peeling or exfoliation of the paint film.